It's very easy to do during the Carolingian era - they had a notional kingdom of Aquitaine containing most of the relevant lands, but the final partition saw it merge with France. Sadly, that's outside the bounds of the PoD.
While the carolingian sub-kingdom of Aquitaine enjoyed a great autonomy (having its own chancery during Louis I reign by exemple), it was still under frankish authority : not only the suzerainty of emperors or carolingian kings, but because of the replacement of local nobility by austrasian (when not relatives of carolingians) rulers, as Gulhèm as count of Toulouse.
The palaces of the kings of Aquitaine were systematically along the Loire river, while the southern marches (Gascony, Marche of Toulouse/Marche of Spain) acted autonomously and more often directly under the authority of emperors than kings (Don't forget that Louis was made king of Aquitaine in his infancy and that Charlemagne expressly asked his tutors to avoid him turning into a "roman").
After the revolt of Pippin II (that was quite limited), Aquitaine even if someties treated as a sub-kingdom (more usually as a titular kingdom though, not unlike Algraves) constitutent of Western Francia rather than distinct.
The duchy of Aquitaine, however, needs to both absorb Toulouse and to win independence from France, and the two goals are somewhat at odds - because of it tries to devour Toulouse, it winds up with nearly half the kingdom and will be extremely feared by other feudal lords, leaving it with few allies to break the power of the French King.
Well, Guilhèm IX tried (and managed sucessfully) to do that in the early XII century, having taken Tolsan (for 20 years) and Rouergue (for 10 years), basing himself on his wife's right being denied by Raimond IV.
The issue is far less having Aquitaine being able to conquer these territories than actually holding them : as you said, it would have encountered many resistences.
- Neighbouring rivals and local nobility, particularly Barcelona, and nobility of conquered lands : OTL the local nobles mainly stiood with Anfòs Jordan (Loyalty being far more due to a dynasty than an abstract and anachronistic cultural unity). Furthermore, Aquitaine was actually divided in many autonomous entities as Auvergne that wouldn't have seen the reinforcement of ducal authority with too much joy. The possibility of having opposers inside the duchy itself were huge.
-Church : Late Ramnulfids had regulars tended relations with churches, often excommunicated or at least taken with great caution.
- King
Not only because of its growing political and military power that allowed him to intervene in all his kingdom, but because his royal title allowed him anyway to crisallise feudal dynamics.
That's eventually the issue with breaking the royal power and rejecting its suzerainty : if the Duke of Aquitaine tried that, his own vassals would have felt authorized eventually to do the same with him. Seeing how much medieval occitan principalities knew feudal division inside their own lands (the size of Aquitaine can be illusory in this regard, but it was a true puzzle), this could have been a bad move.
Basically, the king was the "pillar" of feudal society : removing it outright could have cause the collapse of the whole edifice. Even during the late X, when Aquitains refused to acknowledge Capetians, they used a Carolingian pretender to royal title to justify it.
So suppose Aquitaine comes at some point to follow full salic law preventing any women from inheriting it
Really unlikely to happen, because of butterflies. OTL, what we call wrongly salic law (it was an interpretation of salic law regarding royal sucession), was the result of a relativly precise process.
First, the female inheritence was rare and exceptionnal even before : the question was if they were able to transmit a title. Without the Artois sucession crisis, the question would be still opened (basically the matter of female sucession or transmission was ruled even before the French sucession crisis, where very few support of Joan of Navarra showed up).
And if Aquitaine manages somehow, to become more autonomous, I doubt salic law would be involved, but or the occitano-roman customs (Brievary of Alaric, by exemple) or the customs of Poitou, both that were more or less blurry and helpless about this precise subject (that is anyway an interpretation and adaptation to the ducal sucession that could appear anyway only if there's a need for this).
Suppose something similar to nationalism develops, suppose it develops the idea of a pan-Occitan state instead of harkening back to West Francia,
Seeing how Occitan, up to the word itself, is a creation of capetian administration, I really doubt that butterflying away the french takeover would allow it to exist nevertheless. You may end with an Aquitain "nationalism", maybe, but certainly not Pan-Occitan (that would have been hard to define anyway, few nationalisms are based only on language).